The invention relates to monolithic catalyst support structures formed of alumina, and particularly to new batch compositions and methods for forming complex alumina structures such as alumina honeycombs that overcome some of the limitations of prior art alumina ceramics. Honeycombs provided in accordance with the invention offer particular advantages as column packings in fixed-bed chemical reactors to reduce pressure drop, control temperature distribution, improve the uniformity of reactant distribution, and increase mass transfer efficiency.
Ceramic honeycomb structures composed of alumina and produced by the extrusion of plasticized alumina powder batches containing appropriate binder constituents are known. U.S. Pat. No. 4,631,267 to Lachman et al. describes a variety of permanent binder materials useful for producing relatively strong honeycombs of alumina at relatively low temperatures.
U.S. Pat. No. 3,983,197 describes the production of alumina catalyst extrudates by extruding, drying, and calcining a mixture formed of finely divided alumina, an alumina hydrosol, and water. The alumina hydrosol is characterized as a source of alumina as well as a binder and/or lubricant that obviates the need for additional extraneous binders or lubricants. An alumina-based cement for binding alumina fibers together into a selected shape is described in U.S. Pat. No. 4,349,637. That patent discloses alumina-based formulations consisting of a mixture of colloidal alumina, aluminum chlorhydrate, water, and hydrochloric or nitric acids.
U.S. Pat. No. 3,975,202 utilizes aluminum hydroxyhalides to form rigid gels from solutions comprising acetate lactate accelerators. U.S. Pat. No. 5,244,852 discloses catalyst support coatings containing molecular sieves such as zeolites provided from slurries comprising the zeolites in combination with selected binders. The binders can be selected from the group consisting of high surface areas aluminas and their precursors. Examples are transition aluminas derived from pseudoboehmite, other hydrated aluminas, hydrolyzed aluminum alkoxides such as aluminum isopropoxide, and aluminum chlorhydrates, with the preferred binder being gamma alumina developed by firing pseudoboehmite to 500-600xc2x0 C.
While alumina honeycombs offer significant advantages as catalysts or catalyst supports in chemical reactors designed for carrying out a number of different catalytic reactions, problems with the manufacture of such supports remain. For example, cracking frequently occurs as the extruded honeycombs are dried and fired following extrusion, this cracking being associated in many cases with the shrinkage that occurs as included in the extrusion batch are removed or consolidated.
Another manufacturing problem arises from the need to provide both high fired strength and high fired surface area in the honeycombs. In general the high firing temperatures needed to enhance honeycomb strength also tend to reduce the porosity and surface area of the alumina materials used to form the product.
The invention provides extrusion batch compositions for alumina honeycombs that incorporate novel binder combinations for improved honeycomb extrusion, drying and firing. The binder combinations include both hydrous aluminum oxide permanent binders and polymeric aluminum salts in various combinations. These combinations impart high strength, high active surface area, and controlled pore size distribution to the final body. Honeycombs produced from these improved extrusion batch compositions can be used as catalyst or catalyst support packings in chemical reactors for a wide variety of chemical, petrochemical, and petroluem refining processes. In addition, the batch compositions themselves offer improved batch rheology for extruded honeycomb manufacturing.
The principal component of the extrusion batch compositions of the invention is alumina powder of controlled particle size. Useful alumina powders include alpha alumina, gamma alumina and any of a number of other transition aluminas that have been produced or comminuted to the required particle size. In general these aluminas are anhydrous materials, i.e., they are substantially free of chemically bound water.
Also included in the extrusion batch is at least one precursor for a permanent alumina binder. By a permanent binder is meant a constituent that remains in the honeycomb after firing and is effective to increase the fired strength thereof. Precursors for permanent alumina binders are well known, families of which include colloidal or other finely divided aluminum hydroxides or oxy-hydroxides. These can be synthetic or naturally occurring aluminum hydroxides and hydrated aluminum oxides, specific examples of which include boehmites and pseudoboemites as well as native aluminum hydroxides such as bayerite and gibbsite.
While batches comprising hydroxy aluminum oxide binding constituents alone can form extruded honeycombs, significant advantages including low firing shrinkage and higher active surface area after firing are obtained if an aluminum salt is included as an additional binding constituent in the batch. While carboxylate, nitrate, sulfate, or chloride salts of aluminum can be used for this purpose, a preferred class of salts includes the polymeric hydroxy-aluminum salts (PAS), a specific example of which is polymeric aluminum chlorhydrate (ACH). It is the combination of these salts with conventional aluminum hydroxide or oxy-hydroxide binder precursors that provides the unexpected improvements in fired honeycomb properties that are observed.
In addition to the permanent binders, the extrusion batch will include a temporary organic binder. Suitable temporary binders act to improve the plasticity and cohesion of the batch during the extrusion process, and to enhance the strength of the green extruded body throughout the drying process. In most cases they are completely or substantially completely burned out of the honeycomb at the firing temperatures required.
A variety of different organic compounds and mixtures are well known in the art to have utility as temporary binders. Cellulosic binders such as methyl cellulose are particularly useful for honeycomb extrusion, but other useful compounds are known, including those described, for example, by S. Levine in xe2x80x9cOrganic (Temporary) Binders for Ceramic Systems, Ceramic Age, 75(1), 39+ (January 1960) and xe2x80x9cTemporary Organic Binders for Ceramic Systemsxe2x80x9d, Ceramic Age, 75(2), 25+ (February 1960).
Also important for securing high fired strength in alumina honeycombs is the use of an aqueous acidic peptizing agent in the extrusion batch. These agents, which are typically introduced as dilute organic or mineral acid additions to the water component of the batch, are thought to generate additional hydrated alumina species that promote stronger aluminum oxide inter-particle bonding during the drying and firing stages of honeycomb manufacture. Acetic acid is the presently preferred agent.
Also provided according to the invention is a method for producing crack-free alumina honeycombs of high strength and high active (pore) surface area. In accordance with that method, an aluminum oxide powder is first selected, that powder generally being a comminuted powder that is either produced in finely divided form or converted to that form by the processing of coarser powders.
The selected alumina powder is blended and mulled with a combination of binders, including an alumina precursor such as an aluminum hydroxide or oxy-hydroxide, a polymeric aluminum salt, and a temporary binder. These are mulled together with water and a selected aqueous acidic peptizing agent, the viscosity of this batch mixture being adjusted by means of the water addition into a range suitable for later extrusion.
The batch mixture thus blended is plasticized by mixing, and is then extruded through a honeycomb die to form a honeycomb extrudate. The extrudate is then dried and fired to provide alumina honeycomb products.
A particular advantage of this process is that, through the inclusion of effective amounts of both hydrous aluminum oxide and polymeric aluminum salt binder components in the batch, honeycomb linear shrinkage during drying and firing can be controlled to less than 10%. Proportionate increases in the production yield of crack-free honeycombs of high strength and high active surface area are thereby secured.